48V @ 5A H-bridge circuit.

Does the attached circuit look sensible?

Particularly interested if anyone spots errors in capacitor and resistor values make sense as I modified the circuit from one found somewhere else without fully understanding it. :o

(The data sheet had every example except the simplest use case which happens to be the one I wanted! But most of it looks the same as here, so I am fairly confident).

Driving a 48V motor that wants 5A to run. Generally run for no more than 30 seconds at a time with minutes to hours between.

Added some extra capacitors and diodes (that appear to be for extra circuit safety) from the datasheet reference designs. Also the resistor and capacitor across the load appear to be of suitable value if I correctly understood the bit in the datasheet on how to calculate those. :slight_smile:

I’m pretty confident here now :slight_smile: but since I intend to release the finished and tested board design files public domain for anyone else who has a use for it, I want it to be the best reasonably possible, so any comments/suggestions are very welcome.

Oh, FYI, the project I am using this h-bridge for is here: http://glenalec.net/projects/bed/ :slight_smile:

That chip cannot really handle 5A, that's the absolute max peak value (certainly not continuous).

You need a bridge with discrete MOSFETs to handle 5A. Perhaps look at the HIP4081A driver which
forms a bridge with 4 n-channel MOSFETs.

(The device on-board D-FETs are 0.3 ohm, ie 300 milliohm. For 5A you want 10 milliohm or
less ideally). 5A at 0.3 ohm is ~8W per switching device, ~15W total, which is massive.

And remember a motor with 5A normal load will have stall current of 30 to 60A, which will
definitely explode that chip.

Thanks for the pointer. That sounds like exactly what I was looking for, but didn't know what search terms to use.

Also have a look at the open source motor controller project, they've experience with such designs and

Hmmm. Would using some of these:


in a h-bridge formation be effective?

You seem intent on xyprobleming us repeatedly.

You have a problem, but don't know how to solve it, so tell us the problem, not your guesses at
how to solve it.

From what I can see you need to drive a couple of linear actuators - so tell us full details of this
hardware and your requirements for controlling them. Do you need speed control for instance?

sorry. Yes. the problem:

Drive two brushed-motor linear actuators that each use 5A @ 48V.

Only need forward-stop-reverse at full speed.

Control from TTL levels (Arduino outputs).

Interesting project.

Here's my suggestion: just use two SPDT relays for each actuator.
Cheap, basically foolproof, least chance of magic smoke escaping.


The actuators might well draw more then 10A at startup. But that $10 relay board is a good starting point.

One mosfet and one DPDT relays per actuator could start/stop/change direction/PWM speed control.
Post#4 here shows the principle.


P.S. one relay contact is drawn in the wrong position.

And you still need free-wheel diodes.

Thanks for all the advice. I have settled on using PCB-mount 120VDC@10A relays (5v drive via BC547 and flyback diodes). Non-latching so the default state is all-off via GND-GND. (I have the circuit up on http://glenalec.net/projects/bed/ --near the bottom-- if anyone is interested)

Something that I did not consider when I made the relay suggestion is what happens when you stop the motor.

Stopping means switching both sides of the motor to gnd. Basically shorting the motor.

That will make the motor stop real fast, and all the kinetic energy will be turned into current.

For more info: Braking a DC brushed motor - Electrical Engineering Stack Exchange

For some applications, this could be a problem. Might be a problem in your application!

I suspect that the motor in those actuators is heavily geared down and will stop quickly without frying the relays. I would suggest testing out the 2 relay circuit on the motor to see if it works acceptably or if melts the relay contacts.

Thanks for the heads-up on that, Daenerys. I ordered relays rated over double my needs in V and A (DC ratings, not AC :slight_smile: ) for normal operation, and according to their datasheet they can handle a 6kV surge, so If that doesn’t do it I am not sure if anything will! :slight_smile:

And yes, you are right that the motors stop very suddenly due to the gearing.

Just a quick heads up to say thanks to everyone and it all seems to be working with the relays.